Known long-haul systems have been implemented using binary modulation formats wherein a single data bit is modulated on a single transmitted symbol. One method of performing binary modulation is known as On-Off-Keying (OOK), wherein the transmitted pulse is turned on and off with the ones and zeros of a data bit stream. On-Off-Keying may be implemented in a variety of well-known formats, such as Return-to-Zero (RZ), Non-Return to Zero (NRZ) and Chirped-Return-to-Zero (CRZ) formats. Generally, in a RZ format the transmitted optical pulses do not occupy the entire bit period and return to zero between adjacent bits, whereas in a NRZ format the optical pulses have a constant value characteristic when consecutive binary ones are sent. In a chirped format, such as CRZ, a bit synchronous sinusoidal phase modulation is imparted to the transmitted pulses.
Phase Shift Keying (PSK) is another binary modulation method known to those of ordinary skill in the art. In PSK modulation ones and zeros are identified by phase differences or transitions in the optical carrier. PSK may be implemented by turning the transmitter on with a first phase to indicate a one and then with a second phase to indicate a zero. In a differential phase-shift-keying (DPSK) format, the optical intensity of the signal may be held constant, while ones and zeros are indicated by differential phase transitions. DPSK modulation formats include RZ-DPSK, wherein a return-to-zero amplitude modulation is imparted to a DPSK signal, and CRZ-DPSK.
When the bit rate of a transmission system is increased, e.g. to 40 Gb/s, transmission penalties may become more pronounced. For higher bit rates, multi-level modulation formats have been attractive due to their high spectral efficiency and increased tolerance to chromatic dispersion and polarization mode dispersion compared with the same line rate as binary modulation formats. In a multi-level modulation format multiple data bits may be encoded on a single transmitted symbol.
A number of multi-level modulation formats are known. Examples of multi-level modulation formats useful for encoding two-bits per symbol include: quadrature phase shift keying (QPSK); differential quadrature phase shift keying (DQPSK) wherein information is encoded in four differential phases; and a combination of amplitude shift keying and differential binary phase shift keying (ASK-DBPSK). Multi-level modulation formats with eight symbol levels useful for encoding three bits per symbol include differential 8-level phase shift keying (D8PSK) and ASK-DQPSK. A combination of quadrature amplitude shift keying and differential quadrature phase modulation (QASK-DQPSK) may be used to provide 16 symbol levels, or four bits per symbol.
Two types of multi-level modulation formats that have been considered for 40 Gb/s long-haul optical transmission systems are return-to-zero differential quadrature phase shift keying (RZ-DQPSK) and polarization multiplexing (POLMUX) RZ-DPSK, which involves sending a pair of 20 Gb/s signals at orthogonal polarizations. Both of these formats achieve 2 bits/symbol modulation.
It has been found that that 40 Gb/s POLMUX-RZ-DPSK outperforms 40 Gb/s RZ-DQPSK by ˜4 dB in a 5,200 km Raman assisted EDFA system with 150 Km repeater spacing and 66.6 GHz channel spacing mainly due to higher nonlinear phase noise tolerance and sensitivity. Unfortunately, however, POLMUX-RZ-DPSK requires a receiver including a polarization controller to track the input random state of polarization for each channel and a polarizer to suppress the orthogonal neighboring channel. Any loss of polarization control may result in a degradation of Q-factor in a POLMUX-RZ-DPSK system.